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Exploring Cosmic History Hidden in Gamma Rays

While the fate of the crippled Compton Gamma Ray Observatory remains uncertain, NASA moved last week to ensure the continued study of the powerful and mysterious radiations that appear to signal the most violent phenomena in the distant universe.

The National Aeronautics and Space Administration selected Stanford University to develop and build the main instrument for the Gamma Ray Large Area Space Telescope, better known by the acronym Glast. The entire project is expected to cost $326 million and lead to a launching of the telescope into Earth orbit in 2005.

With a wider field of view and more than 50 times the sensitivity of comparable space-based telescopes, the new observatory should give astronomers their best view yet of the titanic forces released when dying stars explode, dense stars collide with one another or with a black hole and the tremendous gravitational power of supermassive black holes accelerates particles to phenomenal energies and kicks them out in blazing jets reaching light-years across space.

Scientists think the telescope may also produce critical discoveries relating to some of the most formidable quests of modern cosmology. Gamma rays coming from the most distant sources could shine a light on the epoch of earliest galaxy formation, a shadowy time in cosmic history that has so far eluded direct observation. And if most of the dark matter that presumably dominates the universe's mass is in the form of hypothetical exotic particles, collisions of the particles would probably create detectable gamma rays.

''One of the things we know is the sky is aglow in gamma rays,'' said Dr. Brenda Dingus of the University of Wisconsin at Madison, a gamma ray astronomer advising the project. ''They represent incredible amounts of energy, are very distant and often flare suddenly. Their variability is beginning to teach us a lot of what's going on in the universe.''

Dr. Elliott Bloom, leader of the particle astrophysics group at the Stanford Linear Accelerator Center, said the new telescope would be sensitive to ''an ideal energy range for seeing thousands and thousands'' of especially powerful gamma ray sources.

Glast is being designed with a sensitivity to detect gamma rays at high energies ranging from 20 million to 300 billion electron volts, and probably much higher. This is to be at least 10 times the capability of a comparable instrument on the Compton observatory, which has detected about 300 such high-energy objects over the last nine years. This particular detector on the Compton, the Energetic Gamma Ray Experiment Telescope, or Egret, has reached the end of its operating life.

Most gamma ray objects can be observed only from spacecraft, above Earth's radiation-absorbing atmosphere. But a new ground-based detector in New Mexico, operated by Los Alamos National Laboratory and eight universities, has been able to record especially powerful bursts of gamma rays, including one registering nearly a trillion electron volts, by seeing the blue light from the showers of particles produced when an energetic gamma ray hits the atmosphere.

The selection of the Stanford accelerator center to develop the Glast instrument reflects the growing alliance between particle physicists and astrophysicists in the investigation of gamma rays, as well as most other fields of cosmology. After all, the particle accelerating machines of today's scientists are puny compared with the natural forces producing some of these radiations at such extreme energies.

The Stanford team, headed by Dr. Peter F. Michelson, includes more than 100 particle physicists, astrophysicists and other scientists from six countries and several American research institutions.

The Department of Energy, which supports particle accelerator research, is collaborating with NASA on the project. The Stanford proposal was chosen over a competing plan by the University of Alabama and the Marshall Space Flight Center in Huntsville, Ala.

''Glast will enable the study of many important cosmological mysteries where particle physics has fundamental relevance, including the mechanisms of cosmic particle acceleration, the physics driving the mysterious gamma ray bursts and the nature of dark matter,'' Dr. Michelson said in a statement announcing the new telescope project.

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The most powerful particle accelerations appear to occur at the cores of some galaxies that are thought to be occupied by black holes, with masses possibly greater than a billion Suns. Although the gravity of such objects is so strong that nothing, not even light, can escape them, material that comes close but avoids being swallowed is accelerated to energies that produce X-rays and even more powerful gamma rays.

What are known as gamma ray bursts are brief flares of intense energy that appear suddenly and randomly in deep space. Some of these bursts, observed by the Compton spacecraft, may come from the most powerful explosions known since the Big Bang, the theorized explosion at the creation of the universe.

The design for Glast is based on recent advances in the detection technology for major particle accelerators and bears no resemblance to what most people think of as a telescope.

Instead of mirrors or lenses, an array of eight columns composed of thin lead foil interleaved with silicon strips acts to register impacts of gamma radiation and determine the location in the sky of its source. Cesium-iodide calorimeters measure the radiation's energy levels. An anticoincidence detector is the instrument's first line of defense, screening out the more abundant background of cosmic rays.

As planned, the spacecraft bearing the instrument is to be launched from the Kennedy Space Center in Florida into a 340-mile-high orbit. Engineers have been asked to design the craft for a minimum lifetime of five years and a goal twice that long.

Gamma ray astronomers emphasized that Glast and two other small spacecraft being planned will not be able to replace the Compton observatory's full capabilities provided by four telescopes. Glast, for example, will only include a more advanced replacement for Compton's Egret instrument. Only the Compton can monitor the entire sky continuously on the lookout for new gamma ray sources.

''Anything we can do to save Compton would be good,'' said Dr. Dingus of Wisconsin.

But the $600 million Compton observatory, the largest American scientific satellite, has been in trouble since one of its three gyroscopes failed in December. The observatory, in orbit since 1991, is still sending data from three of its main telescopes, all except Egret, and the other gyroscopes orienting the craft show no signs of failing.

But under mission rules, flight controllers may be ordered to send the spacecraft on a controlled suicidal plunge back to Earth, rather than wait and risk the loss of one or both of the other gyroscopes. In the latter event, the concern is that when the 17-ton vehicle eventually falls through the atmosphere, parts of it could land on populated areas. A decision on what to do, originally expected by the end of February, has been delayed for another month.

Dr. Donald Kniffen, NASA's deputy program scientist for the Compton project, said engineers at the Goddard Space Flight Center in Greenbelt, Md., were still evaluating procedures to bring the spacecraft safely down even with the failure of all the gyroscopes.

''There are discussions about bringing Compton in,'' Dr. Kniffen said, ''but also considerable support for leaving it up there, if it can be done safely.''